DOM meta system

This article provides information about the COLLADA DOM meta system.

Element descriptions

Each daeElement has a non-static pointer to its type information: _meta.

Each strongly typed class inherits from daeElement and has a static pointer to the type information: _Meta.

Each element can have its own element name. If getElementName returns NULL then the element’s name is the same as its type name (its daeMetaElement’s name ).

Reflective Object model

The daeElement class is the base class for all strongly typed elements and xs:group elements.

daeElement has storage for very little data. The strongly typed subclasses provide storage for the data that is needed.

daeElement is the base class for reference counted objects. This should be changed, though, as it makes other reference counted objects overly complex and bloated.

The meta system allows access to a strongly typed subclass’s data given a daeElement base class object by storing pointer offsets from the daeElement pointer to the data. This is how all of the “dae” library functions and the libxml2 IO plugin access an element’s data.

daeMetaElement and daeMetaAttribute

daeMetaElement is the main class for an element’s type description.

It stores daeMetaAttributes, and their variations (more later), for each attribute, value, element child, and some auxiliary data (like the _contents arrays)

It stores a tree of daeMetaCMPolicy objects that dictate the behavior and ordering of an element’s children.

daeMetaAttribute stores type information of the data an element contains.

The value of an element is just a daeMetaAttribute which is kept separately from the attribute list.

daeMetaAttribute is the base class for daeMetaAttributeArray. These two classes represent single values and array of values respectively.

daeMetaElementAttribute and daeMetaElementArrayAttribute are types of daeMetaAttributes where the value is another element instead of a data type.

daeMetaElementAttribute and daeMetaElementArrayAttribute also inherit from daeMetaCMPolicy and are part of the content model tree.

daeMetaElement stores the pointer offset to the data that it describes.

getWritableMemory returns a raw (byte) pointer to the data. This pointer needs to be cast appropriately to be used.

Any type that has URI data will be marked as _needsResolve. The metaAttributes for the data will be stored in the _resolvers array.

Calling resolve on a metaAttribute in turn calls resolve on the atomic type for the data the attribute describes. Only resolver and idresolver (uri and idref) types implement functionality for resolve.

daeMetaCMPolicy

daeMetaCMPolicy and its subclasses were added to control the ordering of children.

Elements that contain complex content models or content models with an xs:choice will have an array (_contents) that stores the order of these children.

Accompanying the _contents array is a parallel array _contentsOrder which stores ordinal values associated with the children.

The base ordinal values for each child are calculated during code generation. The actual ordinal value given to a child is calculated at runtime.

Children with a higher ordinal value get placed after children with lower ordinal values.

placeElement, removeElement, findChild, and getChildren are all recursive functions where the behavior is defined by the subclasses.

The subclasses are named after the schema content model type that they are modeled after. i.e. daeMetaSequence represents an xs:sequence group.

daeMetaGroup is slightly special since it contains a daeMetaElementAttribute that represents the group daeElement that needs to be placed. It has placing logic to place the group element in the parent element be then defers placing the child to the group element.

daeMetaChoice requires some additional per object data. This data is stored as a char array in daeElement. Each choice is numbered and the value in the array at the numbered index represents which “choice” has been chosen for its children. A -1 represents that no subtrees have been populated yet.

Particle type descriptions

daeAtomicType is the base class for all particle type descriptions.

The code generator can generate new types. They are registered in the registerDomTypes function in domTypes.cpp.

The process is straight forward that it first searches for the type that the new type is based on. If it finds one it will add it’s name to the binding list. If not then it creates a new rawRef type (I don’t think this every actually happens)

Enum types always generate a new type. The enum type also stores parallel arrays for the values and their string equivalents.

The daeResolverType represents types of daeURI. The memoryToString and stringToMemory functions currently are where special characters are handled. Memory to string is also where there is logic to output only URI fragments instead of full URIs for same document references.